Antibacterial Activity of Oregano (Origanum vulgare L.) Essential Oil Vapors against Microbial Contaminants of Food-Contact Surfaces

Abstract

The antimicrobial effect of eight essential oils' vapors against pathogens and spoilage bacteria was assayed. Oreganum vulgare L. essential oil (OVO) showed a broad antibacterial effect, with Minimum Inhibitory Concentration (MIC) values ranging from 94 to 754 µg cm^-3 air, depending on the bacterial species. Then, gaseous OVO was used for the treatment of stainless steel, polypropylene, and glass surfaces contaminated with four bacterial pathogens at 6-7 log cfu coupon^-1. No viable cells were found after OVO treatment on all food-contact surfaces contaminated with all pathogens, with the exception of Sta. aureus DSM 799 on the glass surface. The antimicrobial activity of OVO after the addition of beef extract as a soiling agent reduced the Sta. aureus DSM 799 viable cell count by more than 5 log cfu coupon^-1 on polypropylene and glass, while no viable cells were found in the case of stainless steel. HS-GC-MS analysis of the headspace of the boxes used for the antibacterial assay revealed 14 different volatile compounds with α-Pinene (62-63%), and p-Cymene (21%) as the main terpenes. In conclusion, gaseous OVO could be used for the microbial decontamination of food-contact surfaces, although its efficacy needs to be evaluated since it depends on several parameters such as target microorganisms, food-contact material, temperature, time of contact, and relative humidity.

Keywords: HS-GC-MS analysis; abiotic surfaces; cross-contamination; foodborne pathogens; natural antimicrobials; oregano essential oil.

Figure 1

Chromatographic profile obtained by HS-GC-MS analysis of OVO vapor phase for MIC 40% at 24 h. Peak identification: 1, α-Pinene (tr = 10.6 min); 2, Camphene (tr = 12.0 min); 3, β-Pinene (tr = 13.5 min); 4, β-Myrcene (tr = 15.4 min); 5, α-Terpinene (tr = 15.9 min); 6, R-Limonene (tr = 16.5 min); 7, γ-Terpinene (tr = 18.0 min); 8, p-Cymene (tr = 18.8 min); 9, o-Cymene (tr = 19.8 min); 10, 1-Octen-3-ol (tr = 23.7 min); 11, Linalool (tr = 26.3 min); 12, Thymol methyl ether (tr = 28.0 min); 13, Thymol (tr = 40.2 min); 14, Carvacrol (tr = 40.8 min).

Figure 2

Experimental set up followed to define the most promising EO vapors to control microbial contamination of food-contact surfaces.

Figure 3

Main steps of evaluation of the antibacterial activity of OVO vapors on food-contact surfaces. (A) St. aureus DSM 799 (a) after growth in BHI broth (b) is diluted in sterile saline solution to OD_600nm = 0.3 (c) and then inoculated over a stainless steel surface (d). (B) the lid of the plastic box is covered by filter paper (e) in order to be loaded with OVO solution far from food-contact surfaces contaminated with the microbial target (f), St. aureus DSM 7799 in the picture (g). (C) Three replicates for each sample (h), and for each food-contact surface, once loaded with OVO solution, are sealed and incubated for 24 h at 25 °C when the vapor phase is sampled at 0, 8, and 24 h of incubation.